1. Field of Invention
This invention relates to the correction of image aberrations caused by x-ray scatter when performing volumetric computed tomography of industrial machine parts.
2. Description of Prior Art
Scatter is a deflection in x-ray direction caused by certain interactions within the target material. This can create a background which corrupts the directly transmitted signal of interest (FIG. 1). The phenomenon is significant in computed tomography of industrial machine parts because metals scatter x-rays to a greater degree, and is especially deleterious for three-dimensional, volumetric computed tomography where the entire object is irradiated by a cone beam of x-rays. This spatially-varying background adds to the true signal and can produce pronounced artifacts when the three-dimensional image of the object is mathematically reconstructed.
The primary measurement data in volumetric computed tomography are sets of two-dimensional x-ray projections taken from various angles with respect to the object. In what follows, it is assumed that a complete volumetric computed tomographic system is available, that the distribution of x-ray intensity in the various projection views has been detected, measured, and digitized in some manner known to the art, and that these two-dimensional arrays of numbers representing the various projection views are accessible for numerical operation by the computer. It is further assumed that the projection arrays are afterwards combined and processed according to the known methods of volumetric computed tomography to produce a three-dimensional density map, or image, of the object.
It is known that the image artifacts caused by scattered x-rays falling on the various projection views can be corrected if the fraction of total signal at each point of every projection caused by scatter is estimated and then digitally subtracted before the projections are combined in the image reconstruction step (FIG. 2).
Until now, the main approach for estimating this scattered component has been by making ancillary measurements using a series of x-ray blocking slits of varying width placed between the object and the x-ray detector. The rationale is that the scattered signal, being incident from a range of directions, can be estimated by extrapolating the series of slit measurements to zero width. However, such a method requires extensive added hardware and provides only a coarse grid of scatter estimates. More importantly, this approach has proven experimentally difficult and unable to provide accurate scatter estimates.
A different approach has involved calculation of the scattered signal from physical first principles using prior knowledge of the object geometry. Accurate scatter estimates are possible in this way using Monte Carlo radiation transport computer codes. However such calculations have hitherto been limited to very simple object forms by the apparent requirement, dictated by universal practice, that the geometry be specified using the standard formulae of three-dimensional coordinate geometry. The fact that such formulae require extensive algebraic manipulation to cast them in a form suitable for digital computation has made this approach prohibitive for all but the simplest models. For this reason, the Monte Carlo method has not until now appeared useful for application to the computed tomography of actual industrial parts.